Who was the British monarch in 1620?

This was James 1st, who came to the throne in 1603- his predecessor, Elizabeth 1st, died unmarried and childless, which brought to an end the Tudor dynasty (which was Welsh in origin).

In those days, Scotland was still a seperate kingdom, so if an English monarch died without a successor then the throne of England passed to the Scottish king. This was James- in Scotland, he was the 6th king to be called James, but only the first one in England, so for that reason he is sometimes known as 'James the 6th and 1st'. James's surname was Stuart, so his reign ushered in the Stuart dynasty.

He died in 1625 at the age of 58, and was succeeded by his son Charles 1st.